Large area alumina ceramic heater

ABSTRACT

A large area heater used, for example, in a laser printer belt fuser or as cooking surface, has an alumina substrate in which two bowed parts of alumina ceramic having opposed concave regions are formed together as a laminate. Electrical resistors are deposited on the laminate. The alumina laminate provides excellent resistance to uneven heating or other thermal stress. Alumina ceramic is readily shaped during manufacture and manufacturing costs and yield are good. A wide variety of large area heaters can usefully employ the laminate.

RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No. 11/007,746filed Dec. 8, 2004 which is a continuation of U.S. patent applicationSer. No. 10/229,369 filed Aug. 26, 2002.

TECHNICAL FIELD

This invention relates to resistive electrical heaters having ceramicsupport or bond, which heat large areas under periodic high thermalstress. Such heaters have many applications such as in toner fusers forelectrophotography, household and industrial cooking surfaces,self-heating pots, and potentially many special-purpose applications.

BACKGROUND OF THE INVENTION

Alumina ceramics are widely used for their general sturdiness and goodconductivity of heat. Compared with similar materials, such as aluminumnitride ceramics, alumina ceramics are very cost effective. Themanufacturing and material costs of alumina ceramics are relativelyinexpensive and provide high yields of the items manufactured.

Prior to this invention, however, alumina ceramics could not be used inhigh thermal stress applications. Such applications involve one part ofthe ceramic being at substantially different temperature than anotherpart. In a toner fuser, high stress occurs when a narrow thick media isbeing fused in a heater wide enough for standard media. The ceramiccracks because the media cools the region of the ceramic to which it isproximate, while the more distant parts of the ceramic are spaced bothfrom the media and a backup support surface and gain temperature. In astovetop-cooking environment, such thermal stress would occur whenrelatively cool water is spilled on part of the burner.

Overcoming this thermal stress weakness of alumina ceramics permits thiscost effective material to be used in a wide variety of applicationswhere it could not previously have been used. During the manufacturingprocess, alumina ceramics can be shaped, for example, in the form of apot, and subsequently fixed in that form. Therefore, the usefulness ofovercoming thermal stress in alumina ceramics is not limited toapplications having flat surfaces.

This invention employs a described alumina ceramic laminate. Laminateceramic printing elements having printed dots of heating elements,typically of size of 0.125 mm in diameter, is known. Since the heatingelements are individual elements, the heating area of each heatingelement is small relative to the surface area of the ceramic support. Apreferred alumina ceramic laminate employed with this invention isobtained from Nikko Company, of Japan, under their product designation,500-459. However, other ceramic laminate materials having thecharacteristics of the alumina ceramic laminate may also be used.

DISCLOSURE OF THE INVENTION

This invention employs an ceramic material having exterior surfaces thatare in compression relative to the bulk ceramic material. In a preferredembodiment, a ceramic material having exterior surfaces in compressionis provided by two bowed parts of alumina ceramic having opposed concaveregions adhered together as a laminate. The two layers may be sinteredtogether under heat and pressure with no identifiable adhesive or anintermediate adhesive layer may be employed. The laminate itself ispreferably substantially flat, but it may be shaped under pressure, forexample to form a pot with a flat bottom and vertical sides, before itis cured to its final condition, which is rigid. Electrical resistiveelements for heating are then applied to the surface of the ceramic in astandard manner, such as by thick film printing.

In another preferred embodiment, a laminate ceramic material is providedby at least three or more ceramic material layers, wherein the outerceramic layers have substantially the same coefficient of thermalexpansion and the center layer or layers has a different coefficient ofthermal expansion relative to the outer layers so that the outer mostsurfaces of the outer layers will be in compression. Alternatively, thecenter layer or layers can have a different firing shrinkage.

The thermal stress resistance of a ceramic laminate is significant inoperation when the heating is substantially uninterrupted over a lengthwhich extends in one direction at least roughly about 5 millimeters (mm)and preferably at least about 15 millimeters. In such large area ofheating, cooling may occur on one area of the heater while another areaof the heater becomes much hotter than the cooled area. Employing thelaminate of this invention, the ceramic heater is highly resistant tothermal cracking, and so current functioning and future use are notjeopardized.

BRIEF DESCRIPTION OF THE DRAWINGS

The details of this invention will be described in connection with theaccompanying drawings, in which

FIG. 1 is a flow diagram of the manufacture of a ceramic laminate inaccordance with this invention;

FIG. 2 illustrates the casting and doctoring operation of themanufacture of FIG. 1;

FIG. 3 illustrates the bowing of a ceramic sheet in the operation of themanufacture of FIG. 1;

FIG. 4 illustrates two ceramic sheets with concave sides opposed whichwill then be pressed together into a laminate in the operation ofmanufacture of FIG. 1;

FIG. 5 illustrates the resulting laminate from the operation of FIG. 1;

FIG. 6 a illustrates a first side of a ceramic heater for a belt fuser;

FIG. 6 b illustrates the opposite side from the side of FIG. 6 a of aceramic heater for a belt fuser;

FIG. 7 is illustrative of a belt fuser employing this invention;

FIG. 8 a and FIG. 8 b are top and bottom views respectively illustrativeof appliance heaters, such as a stovetop heater; having a cookingsurface on its upper side.

FIG. 9 illustrates a flat heater with both heater resistors and theircontacts of the same side for applications such as space heaters, dryerheater and the like.

FIG. 10 is an orthogonal view illustrative of a self-heated potemploying this invention; and

FIG. 11 is an exploded perspective view illustrating a combined heaterand electrical controls with a heat sink at the electrical controls.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The substrate processing flow is shown in FIG. 1. Raw materialconsisting of alumina (aluminum oxide—Al₂O₃) powder such asSumitomokagaku AL-43, Alcoa A-14, Pechiney P-122; low sodium aluminumpowder, each with an average particle size of approximately 3 microns,is blended with Magnesium Oxide (MgO), Silicon Dioxide (SiO₂), CalciumCarbonate (CaCO₃)), binders, plasticizers, and a solvent to form aslurry. Generally, the beginning slurry composition for 96% aluminaconsists of: 60-70 wt. % alumina powder, 2-4 wt. % sintering flux (MgO,SiO₂, and CaCO₃—each having particle sizes of approximately 2 microns),3-6 wt. % of acrylic resin based binder such as PVB (poly vinyl butyl),1-5 wt. % of plasticizer such as DBP (dibutyl phthalate) or DOP (dioctylphthalate) and 20-30 wt. % of solvent such as toluene. The slurrycomposition is then degassed by placing slurry into a vacuum tank, whichhas 70-76 cm Hg of vacuum, for 5 hours.

The degassed slurry or “slip” preparation is formed into a sheet bydispensing the “slip” onto a carrier tape. The carrier tape may becellulose acetate, MYLAR polyester or TEFLON fluorine polymer asdescribed in U.S. Pat. No. 3,991,149. In the example described in thisapplication, PET (polyethylene terephthalate) is the chosen carrier tapematerial. An adjustable doctor blade is used to control the resultingsheet thickness, along with the feeding speed of the carrier tape. Afterbeing formed by the doctor blade, the resulting sheet of slurry or“slip” preparation is dried while on the carrier tape. The typicaldrying condition is 100 deg. C./1 hour. After drying, the resultantsheet, now said to be in the “green state” is rolled into a reel, whilethe carrier tape is also rolled onto a take-up reel.

The doctor blade process is superior to other processes, such asextrusion, in terms of overall uniform density and the number and sizeof pores in the resulting ceramic substrate. However, doctor bladeprocessing is also subject to differences in resultant density from topto bottom of the sheet. Sheets created by the tape casting methoddescribed have a higher density on the doctor blade side (top side ofsheet) than on the carrier tape side (bottom side of sheet). This occursbecause the doctor blade reorients some of the alumina platelets duringforming of the sheet, creating a lower porosity adjacent the doctorblade. Furthermore, because the sheet casting is heated during formingfrom the carrier tape side, higher shrinkage occurs on that side (bottomside). The result is a bowing or camber in the sheet, with the center ofthe sheet being higher than the edges. See FIG. 2 and FIG. 3.

The bow or “camber” created by the doctor blade tape casting methodintroduces differences in physical strength characteristics on the topand bottom surfaces. The doctor blade surface becomes compressed whenthe sheet is flattened. The carrier tape surface is stretched or placedunder tension when the sheet is flattened. Compression and tension bothaffect how resistant the material is to breaking under either mechanicalor thermal stress. A surface in tension is more likely to crack understress than a surface in compression. Surface compression increasesmechanical strength and thermal shock resistance over either a surfacefree of tension or compression, or a surface under tension.

The two layer laminate substrate described optimizes the strengthcharacteristics of the resultant substrate by bringing both outersurfaces into compression as shown in FIG. 4 and FIG. 5. The laminationprocess is accomplished by stacking the green tape with the bottom layerhaving the bowing or camber directed upward and the top layer having thebowing or camber directed downward, thus having the concave sidesopposed. This stacked green tape is heated in that status to 80 deg. C.while held flat under 80-120 kg/sq. cm of pressure for 2-5 minutes inorder to form the lamination. The plasticizer contained in the greentape is selected to have a glass transition temperature range of 0-20deg. C. for optimal lamination strength.

A two layer ceramic material made according to the process describedherein has an overall thickness of about 2 millimeters. Thicker ceramicmaterials may be provided by pressing together multiple two-layerceramic materials or by providing three or more layers of ceramicmaterials wherein the outer layers have substantially the samecoefficient of thermal expansion and the inner layer or layers a lowercoefficient of thermal expansion so that in operation the outer surfacesof the outer layers are put in compression by the slower expansion rateof the inner layer or layers. Alternately, thicker ceramic materials maybe provided by pressing together multiple two-layer ceramic materials orby providing three or more layers of ceramic materials wherein the outerlayers have substantially the same firing shrinkage and the inner layeror layers a higher firing shrinkage so that in curing the outer surfacesof the outer layers are put in compression by higher shrinkage of theinner layer or layers.

It is contemplated that the outer ceramic layers are provided by thesame material, however all layers may be different materials, providedthe outer layers have substantially the same thermal characteristics.

The laminated substrate is punched in the green state, forming sectionsof green tape of a suitable size for firing. A typical punched size forthis application is 300 mm×100 mm. The punched, “green” pieces oflaminated substrate are then sprayed with alumina powder in order toprevent sticking between pieces and fired in a furnace at 1600 deg. C.for 30 Hours. This firing sinters the substrate pieces. The pieces arethen fired at 1300 deg. C. for 45 hours. This second firing serves thepurpose of reducing the amount of curvature or camber in the pieces. Awater spray is used to remove the alumina powder from the surface of thefired substrates. Between firings, red dye is used to detect cracks inthe substrates. The dye is removed in the second firing process.

For applications such as heaters, an additional process of laserscribing (with CO₂ laser) is typically performed. Honing, and annealingmay also be performed as a means of reducing micro-cracks created by thelaser scribing.

Laser scribing is performed using a CO₂ laser set to the following:

Frequency: 400 Hz

Pulse: 200 micron

Speed: 100 mm/sec

Power: 140 Watts

Depth: 700 micron

In this example, the fired substrate is scribed into 9 heater substratearray patterns.

Micro-cracks produced by laser scribing may be reduced by honing; usinga water spray delivered during a buff-grinding operation and containing400 mesh (100-300 micron particle size) alumina powder. The honing sprayis delivered at 400 mm/sec or for 5 minutes.

Micro-cracks can also be reduced further by annealing the laser-scribedsubstrates in a furnace at 1400 deg. C. for 40 hours. Annealing alsoremoves permanent stresses from the substrate.

The amount of camber and other dimensions are checked followingannealing. Visual inspection is also performed. Heater Printing ProcessFlow Table: Process Specification/Supplier 96% Alumina SubstrateThickness = 2.0 ± 0.16 mm Work Size: 285.0 ± 0.1 × 89.2 ± 0.1 mm (9 uparrays) Conductor (Back) Print Ag/Pd: Thickness = 12 ± 2 micron TanakaTR 4865 Conductor (Front) Print Ag/Pt: Thickness = 12 ± 2 micron DupontD 5164N Resistor Ag/Pd Low ohm resistor: 100 m ohm (Tanaka TR 9100) + 40m ohm (Tanaka TR 9040) Resistance adjust Target resistance: 80 m ohm(Blend) Target TCR: 182 ppm (from room temp. to 190° C.) PreliminaryTest Resistance: 13.6 ± 0.7 ohm @ Room Temperature Resistance: 14.0 ±0.7 ohm @ 190 C. Resistor Print Thickness = 10.5 ± 1.5 micron ResistanceTest (1) Resistance: 13.6 ± 0.7 ohm @ Room Temperature Glass(Insulation) Thickness = 35 ± 5 micron (Total 3 layers: Print Dry Fire +Print Dry Print Dry Fire) Glass (Top) Thickness = 10 ± 2 micron (1layer) Resistance test (2) Resistance = 12.85 ± 0.7 ohm @ RoomTemperature Isolation resistance: 100 M ohm @ 250 VDC Visual InspectionPits/Dents/Scratches/Registration/Contamination etc Ag EpoxyEpo-TEK/E3082 or E3084 Mount (Thermistor) SEMITEC/364FT-3P Cure SilverEpoxy Minimal cure 150 deg. C. for 30 minutes Encapsulant GE Toshiba/TSE326 Silicone Cure Encapsulant Cure in air oven for 1 hour at 150 deg. C.Encapsulation Approved encapsulant Singulation Unit Size: 259.5 +0.5/−0.1 × 8.8 + 0.5/−0.1 mm Camber: Max. 0.3 mm

Heater Printing Materials Table: Application Supplier Part #Specification Remarks Substrate NIKKO TBD Thickness: t = 1.50 ± 8% mmWork size = 89.2 + 0.1 mm × 285 + 0.1 mm (9 ups) Unit Size: 259.5 × 8.8mm Tolerance: ±0.2 mm Conductor DuPont D5164N Application: Front MetalBody: Ag/Pt Sheet Resistivity: Typical 3 m ohm at 10 micron FiredThickness: 16-18 microns Tanaka TR4865 Application: Back Metal body:Ag/Pd = 70/30 Sheet Resistivity: Less than 35 m ohm at 10 micron DriedThickness: 17-19 microns Resistor Tanaka TR9040 Sheet Resistivity: 40 mohms at Blend: 10 micron 9040:9100 = 1:4 Resistivity tolerance (Batch toBatch): Less than ±30% Deviation of resistivity: Less than 10% Firedthickness: 11 + 2 micron Hot TCR: 350-450 ppm/deg. C. Cold TCR: 350-450ppm/deg. C. TR9100 Sheet Resistivity: 100 m ohm at 10 micron Resistivitytolerance (Batch to Batch): Less than ±30% Deviation of resistivity:Less than 10% Fired Thickness: 11 ± 2 micron Hot TCR: −50-50 ppm/deg. C.Cold TCR: 0-90 ppm/deg. C. Glass Asahi AP5707 Application: Insulation 3layer print Glass TCE: 7.5-8.1 ppm Annealing Point: 445-465 deg. C.Crystallization Point: 870-900 deg. C. Insulation Resistance: Greaterthan E12 ohm Breakdown Voltage BDV: Greater than 1000 VDC (at less thanthickness used) AP5349 Application: Surface Roughness 1 print layer TCE:5.9-6.5 ppm Annealing Point: 650-670 deg. C. Surface Roughness (Rz):Less than 0.8 Dielectric withstanding: Greater than 1.5 KVACHeater Printing Process Description

The heater can be created on the ceramic substrate by either thick filmprinting or thin film deposition. If thin film deposition is used,either the top surface must be glazed or 99.6% alumina must be used inorder to improve the surface roughness from about 1 micron Ra to about0.15 micron Ra. Firing is typically done in a muffle furnace to separatecombustion gases from the ceramic.

For the thick film printing described in this example, silver palladiumpaste and silver platinum paste are used to print the conductorpatterns. The conductor on the front side (resistor side) of the aluminasubstrate is printed using DuPont D5164N paste, a silver platinumconductor having a typical sheet resistivity of 3 milli-ohms per squareat 10 microns. The material has a viscosity of 275+/−35 Pa s. Theprinted pattern is achieved using a 200 mesh screen and a “squeegee”force of about 1.0-2.0 Kg/cm² at a printing rate of about 300 mm/sec.The printed wet thickness is about 21-22 microns, but is reduced to16-18 microns after drying and firing. Drying (settling) is performed at25 deg. C. for 5-10 minutes followed by a drying oven bake at 150 deg.C. for 10-15 minutes. Firing is performed at a peak temperature of 850deg. C. for a 30 minute to 1 hour firing schedule. A typical 30 minutefiring profile consists of a firing rise rate of approximately 95 degC./minute from room temperature (25 deg. C.) to 500 deg. C., a firingrise rate of approximately 70 deg. C./minute from 500 deg. C. to a peaktemperature of 850 deg. C., holding at the 850 deg. C. peak temperaturefor 10 minutes, a firing descent rate of approximately 70 deg. C./minutefrom 850 deg. C. to 500 deg. C., and a descent rate of approximately 95deg. C./minute from 500 deg. C. to room temperature (25 C.).

The printed conductor pattern on the backside of the heater (thermistorside) is completed using Tanaka TR 4865 silver palladium paste. Thismaterial has a sheet resistivity of less than 35 milli-ohms per squareat 10 microns and a viscosity of 350 Pa.s. It is printed at a wetthickness of about 21-22 microns, using a screen of 250 mesh and asqueegee force of about 1.0-2.0 Kg/cm², at a printing rate of about 300mm/sec. The thickness of this layer is 17-19 microns after drying andfiring. As with the front side conductor, drying (settling) is performedat 25 deg. C. for 5-10 minutes, followed by a drying oven bake at 150deg. C. for 10-15 minutes.

Firing is performed at a peak temperature of 850 deg. C. for a 30 minuteto 1 hour firing cycle. A typical 30 minute firing profile consists of afiring rise rate of approximately 95 deg. C./minute from roomtemperature (25 deg. C.) to 500 deg. C., a firing rise rate ofapproximately 70 deg. C./minute from 500 deg. C. to a peak temperatureof 850 deg. C., holding at the 850 deg. C. peak temperature for 10minutes, a firing descent rate of approximately 70 deg. C./minute from850 deg. C. to 500 deg. C., and a descent rate of approximately 95 deg.C./minute from 500 deg. C. to room temperature (25 deg. C.).

The resistor material for the application described is a blended mixtureof material such as Tanaka TR9040 silver palladium paste and TanakaTR9100 silver palladium paste. The Tanaka TR9040 silver palladium pastehas a sheet resistivity of 40 milli-ohms per square at 10 microns. Theviscosity of TR9040 is 275 Pa-s. The Tanaka TR9100 silver palladiumpaste has a sheet resistivity of 100 milli-ohms per square at 10 micronsand a viscosity of 275 Pa-s. The material is blended by using aplanetary stirring mixer for 1 minute with 2,000 rpm rotation in adedicated jar.

The resistors are printed at a wet thickness of about 22-25 microns,using a screen of 250 mesh, and a squeegee force of about 1.0-2.0Kg/cm², at a printing rate of about 300 mm/sec. The thickness of the twoprinted resistors is 10-12 microns after drying and firing.

As with the conductors, drying (settling) is performed at 25 deg. C. for5-10 minutes followed by a drying oven bake at 150 deg. C. for 10-15minutes. Firing is performed at a peak temperature of 850 deg. C. Atypical firing profile is about a 47 minute profile consisting of afiring rise rate of approximately 70 deg. C./minute from roomtemperature (25 deg. C.) to 850 deg. C., holding at the 850 deg. C. peaktemperature for 10 minutes, a firing descent rate of approximately 40deg. C./minute from 850 deg. C. to 200 deg. C., and a descent rate ofapproximately 20 deg. C./minute from 200 deg. C. to room temperature (25deg. C.).

A glass insulation layer is printed such that it completely covers thefired resistors and also covers the shorting conductor printed betweenthe parallel resistors (completing the circuit of the resistors). Onlytwo electrode conductor lands are left uncovered by glass on the topsurface. These are left exposed for making contact with terminal stripsused to deliver the applied voltage to the heater. No glass is appliedto the backside conductor traces. These traces are used to supplyvoltage to a chip thermistor subsequently.

The insulation glass in this example is Asahi glass paste AP5707 havinga thermal coefficient of expansion (TCE) of 7.5-8.1 ppm, annealing pointof 650-670 deg. C., an insulation resistance greater than E12 ohm, andhaving a breakdown voltage greater than 1000 VDC at a thickness lessthan that printed. The wet thickness is printed in three layers of about20 microns thick each. The first layer is printed, dried, and firedusing the following schedule: drying at 25 deg. C. for 5-10 minutesfollowed by a drying oven bake at 150 deg. C. for 10-15 minutes. Firingoccurs at a peak temperature of 850 deg. C. for a 30 minute to 1 hourfiring cycle.

A typical 30 minute firing profile consists of a firing rise rate ofapproximately 95 deg. C./minute from room temperature (25 deg. C.) to500 deg. C., a firing rise rate of approximately 70 deg. C./minute from500 deg. C. to a peak temperature of 850 deg. C. holding at the 850 deg.C. peak temperature for 10 minutes, a firing descent rate ofapproximately 70 deg. C./minute from 850 deg. C. to 500 deg. C., and adescent rate of approximately 95 deg. C./minute from 500 deg. C. to roomtemperature (25 deg. C.). The fired thickness is about 12 microns.

The second layer is printed, dried according to the above dryingschedule, and then, without firing, the third layer is printed. Thethird layer is dried and fired according to the above schedule.

The cover glass paste used is Ashai glass paste AP5349, having a TCE of5.9-6.5 ppm, an annealing point of 450-470 deg. C., and a dielectricstrength of greater than 1.5 KVAC. This glass is printed in one layer,having a wet thickness of about 16.5 microns. The glass is dried andfired according to the schedule listed for the insulation glass. Thedried thickness is 8-10 microns.

A chip thermistor, such as Semitec 364 FT, is applied to the backside ofthe heater. This is accomplished by dispensing an electricallyconductive epoxy such as EPO-TEK E3082 onto the printed terminationelectrodes of the silver palladium conductor traces. The chip thermistoris placed such that the contacts of the chip thermistor are bonded tothe printed electrodes by the electrically conductive epoxy. The epoxyis cured in an oven at 150 deg. C. for 30 minutes. A siliconeencapsulant, such as GE Toshiba TSE 326 silicone, is dispensed over thethermistor/electrode attachment and cured in an oven for 1 hour at 150deg. C.

The heaters are singulated by snapping through the laser-scribed edgesof each individual heater. This may be done with the use of a fixture.

FIG. 6 a and FIG. 6 b show top and bottom views of a belt fuser heaterformed in accordance with the foregoing procedures. The ceramicsubstrate 1 is substantially flat, although it is formed as laminationof the bowed layers as described. Resistive traces 3 a and 3 b asdescribed are formed on the top and each have a continuous length of atleast about 5 millimeters, more preferably at least about 15millimeters, and preferably extend straight and in parallel at leasteight inches so as to be consistent with the width of ordinary papers.Although resistors 3 a and 3 b are shown to have a continuous connection3 c on one end, connection 3 c is typically conductive, but notresistive so as to avoid a concentration of heat at one end of theheater.

FIG. 6 b shows the thermistor 5 and conductive traces 7 a, 7 b withcontact pads 9 a, 9 b. Thermistor 5 is supported on ceramic substrate 1and thereby responds to the temperature of the substrate 1. As isconventional, this provides an electrical status that is sensed byconductive traces 7 a, 7 b and transmitted through contact pads 9 a, 9b.

FIG. 7 shows a belt fuser using a heater of this invention, such as thatof FIG. 6 a and FIG. 6 b. A fixing film in the form of an endless beltis designated by reference number 11. Pressing roller 12 consists ofshaft 12 a, typically formed from steel, aluminum, or similar metal;a-rubber elastic layer 12 b made of silicon rubber, and surrounded byparting layer 12 c, typically consisting of a fluoro polymer sleeve.

Pressing roller 12 is urged to the bottom surface of heater 6 by aresilient member or other urging means (not shown). The bottom travelportion of belt 11 is interposed between heater 16 and pressing roller12. Roller 12 is driven by an attached gear (not shown) throughconnection with a series of gears to the gear train of a printer (notshown). Movement of belt 11 is in a clockwise direction and is driven bypressing roller 12, thereby moving media P in the correspondingdirection through the nip formed by belt 11 and pressing roller 12.

Belt 11 is an endless tube, which is rotated by contact with the drivenpressing roller 12 repeatedly for fixing a toner image. Belt 11 istherefore made of a highly heat resistive and durable material havinggood parting properties and a total thickness of not more than about 100microns, preferably less than about 65 microns. The body of belt 11 is apolyimide resin or the like.

To facilitate parting of media P, leaving toner on media P, belt 11typically has an outer layer (not separately shown) of low surfaceenergy material, such as one or a blend of similar fluoropolymers. Onthe lower, opposite surface of belt 11, to the surface of heater 16, alayer of high viscosity lubricant or grease (not separately illustrated)is applied to lubricate the inner surface of the belt.

Heater 16 comprises the ceramic substrate 13, extending in a directionsubstantially perpendicular to the direction of movement of belt 11.Substrate 13, being ceramic, is electrically insulative, has a highthermal conductivity, has low thermal capacity, and has high heatresistance. As discussed in the foregoing, substrate 13, being inaccordance with this invention, has exceptionally high heat resistanceto thermal stress.

One or more heat generating resistors 15 extend along the length ofsubstrate 13 on the lower surface of substrate 13 (i.e., along the faceof heater 16 which directly contacts film 11), and a thermistor or othertemperature detecting element 14 is mounted in contact with the backface of the substrate 13 (opposite the face having heat-generatingresistors 15). Heater 16 is fixed to a holder 17 with the bottom face ofheater 16 facing the nip that receives media P.

Media P carrying toner is supplied to the fixing device guided by aninlet guide 19, and is introduced into a nip N (fixing nip) between thetemperature-controlled heater 16 and pressing roller 12, moreparticularly between fixing belt 11 and pressing roller 12. Media P ispassed through fixing nip N at the same speed as belt 11 is moved withthe surface of media P having an unfixed toner image Ta being contactedwith the bottom surface of belt 11, which is moving in the samedirection as media P. Tb is toner in nip N. Loose toner Ta is fixed ontomedia P, such as paper, to form fixed toner Tc.

Alternative Applications

This invention applies to heaters formed on a substrate having one ormore continuously printed resistor traces. Each resistor trace has acontinuous length of at least about 5 millimeters and preferably about15 millimeters or more. Similarly conductor traces may be printed inorder to complete the circuit and provide contact points for externalconnectors. The resistor patterns may be in the form of strips printedin a straight line or in a variety of geometric shapes and patterns, asfurther illustrated by the following examples.

FIGS. 6 a, 6 b through 11 illustrate various heater applications using atwo laminate substrate alumina heater. More than one resistor trace maybe printed as shown in FIG. 6 a and FIG. 11, or a single resistor tracemay be printed as shown in FIGS. 8, 9 and 10. Conductor traces may beprinted and used for connecting external electrical connectors forsupplying voltage to the resistors or connecting resistor tracestogether. Also, conductor patterns may be printed for supplying DCvoltage to a thermistor or to other electronic circuitry.

Alumina substrate heaters have the advantage of quick warm-up timescompared to conventional heaters due to their relatively low thermalmass. They have the additional advantage of relatively fast and accuratetemperature control with the use of a variety of direct contactthermistor types. For example, a thermistor may be applied in chip formsuch as Semitec device 364-FT (using an electrically conductive epoxyapplied directly onto the laminated substrate surface) for relativelyfast and accurate temperature measurement and control. In otherapplications, an external thermistor, such as a Semitec HF-10122 sensor,may be placed in direct contact with the laminated substrate surface.

FIG. 8 a and FIG. 8 b show an example of one possible form useable forappliance heater applications. Such a heater might be used as either astovetop heater for direct contact with cooking vessels or as a built-inheater for small appliances such as a teapot, rice cooker or the like.Resistor trace 20 is a continuous expanding spiral on the top surface(FIG. 6 a) of a round, ceramic lamination 22 in accordance with thisinvention. The largest width of the spiral of resistor trace 20 is at intwo perpendicular (orthogonal) directions is at least 10 millimeters toaccommodate standard cooking utensils.

In the particular arrangement shown, the electrical connectors 26 a and26 b (FIG. 8 b) for the voltage supply might be made on the backside ofthe heater. In such an arrangement, the substrate 22 has holes withconductor traces printed such that the conductor material is continuousfrom the top conductor through the hole (“via”) to the bottom conductor.

FIG. 9 illustrates a heater type that might be used in a variety ofheating applications such as coupled with a fan for a space heater,drying cabinet heater, or used in a small appliance such as a waffleiron or clothes iron. The pattern and size could be changed to bestaccommodate the particular application.

The substrate 30 of FIG. 9 is the laminated ceramic of this invention.Substrate 30 carries on one side a resistor trace 32 in a sinusoidalpattern to widely distribute the heat, and conductive contacts 34 a and34 b on the same side of substrate 30 as the resistor 32.

FIG. 10 shows a heater example of a vessel 40 of one piece forming aself-heating pot, such as a teapot, coffeepot, rice cooker and the like.The shape and size of the heater could be adjusted to match theparticular needs of the vessel. FIG. 10 shows a shallow vessel 40 so asnot obscure to elements of particular interest. Vessel 40 has a circularbottom 40 a and a substantially cylindrical side 40 b generallyperpendicular to bottom 40 a.

The vessel of FIG. 10 is made entirely of a single ceramic laminatesubstrate 40 of this invention, carrying on the inner bottom 40 a aresistor trace 42 forming a circle almost completely around bottom 40 ato distribute the heat across the bottom. As discussed in the foregoing,the ceramic laminate is flexible until cured. Prior to curing inaccordance with the embodiment of FIG. 10, the laminate is formed intothe form of a bottom with sides and cured.

FIG. 11 shows a type of hybrid board 50 of the ceramic laminate of thisinvention having a heater circuit 52 and control circuitry 54 on thesame support 50. Such arrangements are another example of the advantagesof thermal shock resistant laminated substrates. In this case, a heatsink 56 is attached proximate to the control circuitry portion of thesupport 50, purposely increasing the gradient between the heated andunheated portions of the support to protect the circuitry 54 fromexcessive heat.

One main advantage of the double layer laminate substrate heater is itsgreatly improved resistance to thermal gradients and thermal shock. Thisapplies to heating applications in several ways. For example, a heaterused as a stovetop burner may undergo significant temperature gradients(on the order of 10-100 deg. C.) by partial coverage of the cookingvessel. The absolute temperature difference created in a heater tracewill vary in direct proportion to the length of exposed heater trace.The heater may also undergo thermal shock, such as that caused by watersplashing from a pot being heated. Laminated substrate heaters have beenshown to withstand repeated thermal gradients on the order of 150 deg.C./mm and thermal shocks such as water shocking when heated to as muchas 400 deg. C.

It is contemplated, and will be apparent to those skilled in the artfrom the preceding description and the accompanying drawings, thatmodifications and changes may be made in the embodiments of theinvention. Accordingly, it is expressly intended that the foregoingdescription and the accompanying drawings are illustrative of preferredembodiments only, not limiting thereto, and that the true spirit andscope of the present invention be determined by reference to theappended claims.

1. A method for forming a ceramic laminate from first and second sheetsof ceramic material, said sheets having first and second surfaceswherein said first surface has a higher density than said secondsurface, comprising: (a) heating said first and second sheet of ceramicmaterial; (b) stacking said first and second sheet of ceramic materialwherein said second surfaces are opposed to one another to providestacked sheets; and (c) pressing said stacked sheets and forming saidceramic laminate.
 2. The method of claim 1 wherein said heating of saidfirst and second sheet of ceramic material comprises heating said secondsurface of said first and second sheet.
 3. The method of claim 1 whereinsaid step of heating said first and second sheet of ceramic materialcomprises heating and forming a bow or camber in said sheet.
 4. Themethod of claim 1 wherein said step of pressing said stacked sheetscomprises heating and pressing said sheets into a flat configuration. 5.The method of claim 1 further including the step of providing aconductor on said ceramic laminate.
 6. The method of claim 1 whereinsaid step of heating said first and second sheet of ceramic materialprovides higher shrinkage to said second surfaces of said sheets ascompared to said first surfaces of said sheet.
 7. The method of claim 1wherein the ceramic material layers comprise alumina ceramic material.8. The method of claim 1 wherein said pressing of said stacked sheetscomprises compressing said first surface of said sheets and stretchingsaid second surface of said sheets.
 9. A method for forming a ceramiclaminate from first and second sheets of ceramic material, said sheetshaving first and second surfaces wherein said second surface has ahigher firing shrinkage than said first surface, comprising: (c) heatingsaid first and second sheet of ceramic material; (d) stacking said firstand second sheet of ceramic material wherein said second surfaces areopposed to one another to provide stacked sheets; and (e) pressing saidstacked sheets and forming said ceramic laminate.
 10. The method ofclaim 9 wherein said heating of said first and second sheet of ceramicmaterial comprises heating said second surface of said first and secondsheet.
 11. The method of claim 9 wherein said step of heating said firstand second sheet of ceramic material comprises heating and forming a bowor camber in said sheet.
 12. The method of claim 9 wherein said step ofpressing said stacked sheets comprises heating and pressing said sheetsinto a flat configuration.
 13. The method of claim 9 further includingthe step of providing a conductor on said ceramic laminate.
 14. Themethod of claim 9 wherein said step of heating said first and secondsheet of ceramic material provides higher shrinkage to said secondsurfaces of said sheets as compared to said first surfaces of saidsheet.
 15. The method of claim 9 wherein the ceramic material layerscomprise alumina ceramic material.
 16. The method of claim 9 whereinsaid pressing of said stacked sheets comprises compressing said firstsurface of said sheets and stretching said second surface of saidsheets.